Except for one group on AC chemotherapy, the study finds chemotherapy did not significantly reduce risk of death in breast cancer patients over the age of 80.
The study, led by researchers from The University of Texas Health Science Center at Houston (UTHealth), is published in the Journal of American Geriatrics Society.
For their study, the team used data from the Surveillance, Epidemiology and End Results (SEER) database covering the period 1992-2009. SEER is run by the National Cancer Institute and provides cancer statistics on the US population.
The data they analyzed came from 14,440 women diagnosed with Stage I to IIIA hormone receptor-negative breast cancer and 26,893 men and women diagnosed with Stage III colon cancer. All the patients were over the age of 65.
For the breast cancer patients, the analysis showed chemotherapy reduced their risk of death from all causes by 30% in those aged 65-69, by 26% in those aged 70-74 and by 24% in those aged 75-79.
But for women over the age of 80, chemotherapy did not significantly reduce risk of death.
There was one exception. In a small number of women over the age of 80 who received combination chemotherapy in the form of doxorubicin (Adriamycin) and cyclophosphamide (Cytoxan) - also known as AC chemotherapy - there was a 29% reduction in death risk.
Chemotherapy effective for all age groups among patients with colon cancer
When choosing a chemotherapy to treat breast cancer, doctors take a number of factors into account, including the patient's age and state of health, and characteristics of the cancer, such as stage, hormone-receptor status, HER2 status and lymph node status.
In contrast to the breast cancer results, the team found chemotherapy appeared to be effective for all age groups over 65 in participants with colon cancer. For example, in those aged 85-89, chemotherapy was linked to a 21% lower risk of death.
The researchers say because of the large sample sizes in their study, it strengthens evidence suggested by previous smaller clinical trials that have found chemotherapy is not effective for breast cancer patients over the age of 70.
Lead author Xianglin Du, a professor of epidemiology, suggests:
"Chemotherapy's reduced effect on the risk of mortality in older breast cancer patients could be due to several factors: tumors being less sensitive to chemotherapy, a decrease in dosage as the body gets weaker with age or chemotherapy killing healthy cells in addition to cancer cell."
Meanwhile, a 1-year trial that Medical News Today reported on recently suggests that doubling the amount of moderate to vigorous exercise from 150 to 300 minutes a week may lower the risk of postmenopausal breast cancer because of the beneficial effect on body fat.